Conventionally, photomultipliers (PMT: Photo-Multiplier Tube) have been known as optical sensors. A photomultiplier comprises a photocathode that converts light into electrons, a focusing electrode, an electron-multiplier section, and an anode, and is constituted so as to accommodate those in a vacuum case. In such a photomultiplier, when light is made incident into a photocathode, photoelectrons are emitted from the photocathode into a vacuum case. The photoelectrons are guided to an electron-multiplier section by a focusing electrode, and are cascade-multiplied by the electron-multiplier section. An anode outputs, as signals, electrons having reached among multiplied electrons (for example, see the following Patent Document 1 and Patent Document 2).
Patent Document 1: Japanese Patent No. 3078905 (Japanese Patent Application Laid-Open No. 5-182631)
Patent Document 2: Japanese Patent Application Laid-Open No. 4-359855